No 45-degree rule written down
Many councils publish a householder design guide with a light test and a maximum depth. We couldn't find one for Portsmouth. The council's list of planning documents has nothing of the kind, so a householder application here is judged against the design policy in the Portsmouth Plan, adopted in 2012, known as PCS23.
That policy asks for scale, layout, appearance and materials that suit the particular context, and for protection of amenity for neighbouring occupiers. It also wants new work to relate to the city's geography and history. Broad words. On a row of island terraces with yards a few metres deep, they come down to one question. How much light and outlook does the house next door lose?
So draw the neighbour's back windows on your plans. An officer can only weigh what the drawings show.
Fitting the work to a Portsmouth house
Single-storey rear. On a terrace or semi you can usually go 3 metres past the original back wall without an application, up to 4 metres high. The larger route, up to 6 metres, starts with an email to the council's householderpa@portsmouthcc.gov.uk address. Neighbours then get 21 days to object and the council decides within 42.
Side return. The narrow passage beside a back addition is where most Fratton and Southsea kitchens find their extra width. Inside any of the city's 25 conservation areas, though, building past a side wall needs planning permission even at one storey.
Kitchen. Where the floor level sits matters more here than in most places. In Flood Zones 2 and 3 the council's flood study wants finished floors above the design flood level, so ask your designer before you fall in love with a step-free threshold onto the garden.
Open plan. Knocking a terrace's back rooms into one means steel sitting on old walls and new pads. The British Geological Survey found the island's ground changing over short distances, so pay for trial holes before the engineer sizes anything.
Two storey. Mostly a job for the larger 1930s semis on the mainland. Under permitted development it can reach 3 metres at the rear if it stays 7 metres from the back boundary. Few island gardens are that long.
Wrap-around. Part side, part rear, so it falls outside permitted development in a conservation area. Elsewhere the side part must stay one storey and no wider than half the original house.
Downstairs bedroom. Adding a bedroom to a family house doesn't trigger the city's nitrate mitigation. If the house is let to sharers, the rules change completely (see the FAQ).
Home office. Extensions and outbuildings together can't cover more than half the land around the house. On a small yard, attach the office and keep the garden.
Planning. Check three things first. Is the house in a conservation area? Is it in Flood Zone 2 or 3? Is it a flat? Flats have no householder extension rights at all, so a converted villa needs a full application.
Cost. We don't publish figures. In Portsmouth the price moves with the ground, the floor level a flood zone demands, and how a builder gets materials through a terrace with no rear access. Get three quotes priced on the same drawings.
Building on a low island
Most of Portsea Island is less than 10 metres above sea level, and the council's 2025 flooding paper says large areas to the south of the island and to the north, mainland included, fall within Flood Zones 2 and 3. It expects those areas to grow as sea levels rise. Spots it names as flood-prone include ground by Hilsea Moat, part of Portsmouth Point and the northern shore of Eastney Lake.
If any part of your extension falls in Zone 2 or 3, the council's validation checklist asks for a flood risk assessment with the application. Short for a householder job, but the floor level it sets feeds into the design. Find your address on the Environment Agency's flood map before the architect starts.
Portsea Island drains through combined sewers to the Eastney pumping station, so a bigger roof adds rain to pipes already carrying foul waste. Ask how the scheme handles it.
What the council asks for with an application
Portsmouth's checklist is worth reading before you brief anyone. A tree report is needed if there are trees on or next to the site. Inside or next to a conservation area or listed building you need a heritage statement, and at the very least it should refer to the Portsmouth Historic Environment Record. Change the front of the house and the council wants street scene drawings at 1:100.
Plans go in at 1:50 or 1:100, and anything stamped 'do not scale' will be turned away. Householder applications also need the CIL additional information form, without which the council may refuse to validate. Small omissions like these hold up more applications than design does.
One reassurance for Southsea owners. The Article 4 directions on streets such as Campbell Road, Chelsea Road and Castle Road control front windows, doors, chimneys, front walls and the front roof covering. None we read touches rear extensions. The conservation area rules still apply on top.
Where these facts come from
- Portsmouth City Council: permitted development rights for household extensions
- Portsmouth City Council: supplementary planning documents
- Portsmouth City Council: The Portsmouth Plan (2012)
- Portsmouth City Council: local validation checklist (2023)
- Portsmouth City Council: Development and Flood Risk Topic Paper (2025)
- Portsmouth City Council: Nutrient Neutral Mitigation Strategy (2022)
- Portsmouth City Council: HMO SPD (2019)
- Portsmouth City Council: Article 4(2) directions, Campbell Road conservation area
- Portsmouth City Council: Article 4(2) direction, Castle Road
- British Geological Survey: Portsea and Hayling resurvey
- GOV.UK: Permitted development rights for householders, technical guidance
How FirQuote works in Portsmouth: We connect you with local specialists who visit your property before giving you a price. You get up to 3 written quotes — no call centres, no estimates from photos, no obligation to accept any quote.
Questions people in Portsmouth ask
We rent rooms to sharers. Can we extend to add bedrooms?
Possibly, but check first. Taking a shared house past six occupants makes it a large HMO, which needs planning permission, and in streets where more than 10% of homes are HMOs the council weighs the extra bedrooms. That kind of enlargement also needs nitrate mitigation. The council's HMO guidance sets minimum room sizes too, such as 6.51 square metres for a single bedroom.
Does a bigger family house need nitrate mitigation?
No. Portsmouth's nutrient strategy says extensions to a family home for extra bedrooms shouldn't need mitigation, because a single household has no upper limit on numbers. New dwellings and HMO enlargements are a different matter.
The council's web page says prior approval works in conservation areas. Is that right?
Treat it with caution. The government's technical guidance says the larger single-storey route isn't available on conservation area land, so a rear extension past 3 metres there needs a householder application. Confirm with the planners before you rely on either.
We're near Hilsea Moat. Do we need a flood risk assessment for a small rear room?
If any part of the work sits in Flood Zone 2 or 3, yes. The validation checklist asks for one.